Electroacoustic transducers are generally comprised of an array of active elements in the form of piezoelectric crystals that are mounted in parallel, spaced relationship on the surface of a base of sound-absorbing material. The base is mounted upon a support structure, typically provided in the form of a heat sink. The base is usually constructed of a backing material that exhibits particular acoustical characteristics. The backing material is typically formed by molding a composition of a material having a high acoustical impedance, such as tungsten powder, and an acoustically-absorbing binder so as to substantially eliminate spurious acoustic reflections.
In constructing such a transducer, it is customary (in some applications) to adhere the back of a large crystal to the surface of the base and saw through it in parallel spaced planes so as to form the separate crystals of the array. Acoustic transducer arrays, and in particular ultrasound transducer arrays, may be arranged in a number of configurations including linear, one-dimensional arrays, matrix two dimensional arrays, annular ring arrays, etc. Harmful coupling between the elements of the array by surface waves is substantially reduced by extending the cuts into the base. The base therefore must be precisely cut and exhibits efficient rigidity so as to maintain the crystals in proper position.
It is, therefore, desirable that the base offer certain mechanical and acoustical characteristics: rigidity, for structural support of the elements in an array; selectable acoustic impedance, for controlling or eliminating the reflections at back surfaces of the elements, to achieve a desired balance between output power and image sharpness; and acoustical attenuation, such that acoustic signals exiting the back of the active elements be substantially attenuated so that image-degrading reflections of such signals are not returned to the transducer element.
Referring to FIG. 1, a schematic of a prior art ultrasound transducer 8 is shown which includes a pulse generator 10 and a matching layer 12 for coupling ultrasound signals into a patient's body. An acoustic absorber backing 14 and a support 15 are positioned behind pulse generator 10. Transducer 8 includes an application face 16 which is placed against the patient's body and from which the principal ultrasound pulses emanate. Pulse generator 10 also propagates pulses through rear face 18 into absorber backing 14. Echoes coming from support 15 are not desired because such echoes appear on the ultrasound display as noise artifacts. As a result, the attenuation rate of absorber backing 14 has to be high to prevent such echoes from appearing on a display screen.
When a pulse generator 10 is energized, a sound signal T is emitted in a forward direction and is reflected by body tissue, whereas a sound signal B is transmitted in the rearward direction through absorber backing 14, reflected by support 15 and redirected in a forward direction. FIG. 2 is a schematic of reflected signal level vs. time and indicates the size of signal T as reflected from the body tissue vs. the size of the signal in absorber backing B as reflected from support 15. The difference in magnitude in signals T and B is achieved by making the attenuation of absorber backing 14 greater than the attenuation of sound in the body. Note that the sound in absorber backing 14 keeps bouncing back and forth between support 15 and pulse generator 10 until it is entirely absorbed.
It has been found, that when support 15 is attached to absorber backing 14, artifacts sometimes appear on the ultrasound display screen during imaging. This is particularly the case when transducer 8 is thin and when heat sinks (which are relatively thick) are used as backing support. A thin transducer is generally desired in order to make the overall transducer smaller and more easily manipulated.
Due to the lessened thickness of absorber backing 14, the round trip attenuation of sound within absorber backing 14 is lower in thin-aspect-ratio transducers as compared to the thicker variety. This causes more sound energy to be available at pulse generator 10 and thereby causes display artifacts. The attenuation level of absorber backing 14 dictates a minimum thickness transducer 8 which can be made without artifacts. It has also been determined that the shape of a rear-attached heat sink, its placement with respect to absorber backing 14 and the method of mounting the heat sink all effect the amount of displayed artifact. It has been thought that such display artifacts were due to mechanical resonance in the transducer structure and, while various changes in geometry and attachment methods between the heat sink and support body 15 have been tried, some display artifact from rear-reflected signals still remains.
Further analysis of the sound reflective characteristics of transducer 8 in FIG. 1, especially when it is configured as a "thin" transducer, indicate a second source of reflected sound (i.e. signal S) which results from reflections from the back of support 15. Signal S is later in time than signal B due to the increased travel distance through support 15.
FIG. 3 is a schematic of signal level at pulse generator 10 as a function of time, considering signals T, B and S. The signal level T from body tissue is the same as described for FIG. 2. The decay rate of signal B from absorber backing 14 is initially slightly higher than that shown in FIG. 2 because some of the initial pulse energy is transmitted into support 15. While signal S is in the support 15 it does not decay with time. Thus, signal S, which comes from the back surface of support 15, decays at a lower rate than signal B (which is entirely in absorber backing 14). This action causes the overall level of signal at pulse generator 10 to decay much more slowly. The knee of curve K corresponds to the time it takes for the first echo S from within support 15 to reach the face of pulse generator 10. That time is proportional to the thickness of acoustic absorber backing 14. The slope of curve portion S, i.e. the decay rate of echoes from within support 15, is determined by the ratio of the thickness of support 15 divided by the thickness of absorber backing 14. Thus, the thicker is support 15 and the thinner is absorber backing 14, the more display artifact is present. The geometry is also important. If support 15 is wider than the backing (as shown in FIG. 1), the slope of S is also reduced.
Sudol et al., in U.S. Pat. No. 5,629,906, disclose an acoustic transducer having a support structure which holds an acoustic pulse generator. An acoustic absorber is attached to the rear face of the pulse generator. An acoustic isolator is positioned between the acoustic absorber and a support structure/heat sink. The acoustic isolator includes at least a first material layer exhibiting a first acoustic impedance value, and a second material layer exhibiting a second acoustic impedance value. The second acoustic impedance value is substantially different from the first acoustic impedance value. A boundary between the first material layer and the second material layer causes multiple acoustic reflections of an acoustic pulse emanating from the rear face of the pulse generator. The acoustic isolator acts as a multiple reflective layer and prevents a substantial percentage of rear propagated acoustic energy from entering and being reflected by the support structure, thereby reducing ultrasound display artifacts.
However, Sudol teaches that each material layer be bonded directly to an adjacent material layer without intervening adhesive or other non-thermally conductive material, and a diffusion bonding process be employed. Such a bond is expensive and difficult to achieve in a consistent and defect-free manner. Furthermore, such a technique can be subject to the inclusion of undesirable air pockets, each of which can reflect acoustic energy and accordingly cause image artifacts. Extensive testing of each assembly may be required to confirm that few or non of such pockets are present.
Accordingly, the advent of ever-smaller ultrasonic transducers continues to impose a need for highly-attenuating base wherein the thickness of the base is reduced. However, it has proven difficult to achieve a base that, in addition to providing adequate structural support, can be constructed to exacting tolerances as a thin member, is highly attenuating, and is easily constructed without defects. Certain transducer arrays are useful for some applications if constructed to dimensions that are more exacting than those made possible by conventional methods.
There remains a need for a thin aspect ratio ultrasound transducer which exhibits excellent heat dissipation properties, is susceptible to formation to close tolerances by use of precision forming techniques, such as high-speed milling or electrical discharge methods (EDM), and nonetheless provides effective attenuation of rear-transmitted acoustic energy.